4-n-Decylphenyl-3'-methyl-4'-(4''-nitro-benzoyloxy)benzoate (10 PMNBB) which exhibits the smectic A and nematic mesophases shows a large expansion (sim20%) of the bilayer spacing as it is cooled in the mesomorphic range. This is accompanied by a reversal in the sign of the dielectric anisotropy which becomes negative at lower temperatures.